1. Field of the Invention
The present invention relates generally to a rocker-arm shaft for internal combustion engines and specifically to an oil passage structure for a rocker-arm shaft rotatably supporting a rocker arm for internal combustion engines.
Description of the Prior Art
As is well known, there have been disclosed various rocker-arm shafts employing an oil lubricating system through which lubricant is supplied to a rocker arm of an internal combustion engine so as to insure a smooth movement of the rocker arm. As seen in FIGS. 1 and 2, a conventional rocker-arm shaft A includes a plurality of axial oil passages 20 and 21 communicated with a main oil gallery and a plurality of radial oil passages 22 communicated with either the axial o passages 20 or 21 for the purpose of oil supply to an oscillating portion of each engine rocker arm (not shown). In such a conventional rocker-arm shaft, axial oil passages 20 and 21 are disposed in parallel with each other to reliably feed lubricating oil to each radial oil passage employed to an oscillating portion of each rocker arm as clearly shown n FIG. 1. As is generally known, it is difficult to provide a high straightness of an axial oil passage by machining. If adjacent axial oil passages are extremely long, there is a possibility that the adjacent oil passages interfere with each other. Moreover, there is a possibility that these axial oil passages penetrate through an outer surface of the rocker-arm shaft. For this reason, there have been disclosed and developed various techniques with regard to an oil passage structure for a rocker-arm shaft. One such prior art oil passage structure has been disclosed in Japanese First Publication Tokkai (Showa) 63-57805. The conventional oil passage structure for a rocker-arm shaft includes a X-shaped or Y-shaped thin-plate partition member press-fitted into a hollow rocker-arm shaft to define a plurality of axial oil passages in the hollow rocker-arm shaft. This conventional oil passage structure can eliminate the aforementioned problem of a high accuracy of machining, such as drilling, required to provide a high straightness of the axial oil passage. However, in the previously described conventional oil passage structure for a rocker-arm shaft employing such a thin-plate partition member provides a relatively small contact area between an inner wall of a hollow rocker-arm shaft and edges of the thin-plate partition member. It is difficult to provide a reliable press-fitting between the partition member and the rocker-arm shaft. This results in oil leakage between adjacent axial oil passages defined by the thin-plate partition member press-fitted into the rocker-arm shaft. In other words, the conventional oil passage structure cannot assure a high sealing characteristics with regard to a partition member and insure reliable oil supply to an oscillating portion of each engine rocker arm. Furthermore, the above noted X-shaped or Y-shaped thin-plate partition member requires a difficult skilled manufacturing process and a hard dimensional tolerance.